Investigation of developed precipitates in AlMgSiCu alloys with and without excess Si

The effect of temperature on the sequence of hardening precipitates in Al-1·15Mg 2 Si-0·34Cu-0·21Cr (wt-%) balanced and Al-1·14Mg 2 Si-0·34Cu-0·21Cr (wt-%) alloys with excess Si has been investigated by hardness measurement (HV), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The values of microhardness which are corresponding to the hardening precipitated particles in the alloy containing excess Si are higher than that in the alloy without Si. The results showed that, the difference in the hardening precipitation peak positions may match the fact that, the excess Si increases the density of β" metastable phase and also, reduces the Mg/Si ratios in the early stage of GP zones and co-cluster formation. After the complete formation of the metastable needle shaped precipitates β" takes place, the strengthening of the alloy would take place as a result of the formation of the semicoherent rod shaped precipitates β' and/or Q' phases.